The present invention relates to an electrophotographic photoreceptor comprising a charge transportation layer composed of a photosensitive material containing a hole mobility material.
An electrophotographic photoreceptor (hereinunder referred to as "photoreceptor") is composed of a conductive substrate of, for example, an aluminum alloy and a photosensitive layer composed of a photoconductive material such as amorphous selenium provided on the conductive substrate. When such a photoreceptor is used for a printer a reverse development system is generally adopted in which positive corona discharge is used for charging and negative corona discharge is used for transfer.
In this system, the photosensitive layer is first positively charged by corona discharge in the dark. A laser beam is then projected onto the surface of the photosensitive layer in correspondence with the image, whereby the potential of the exposed portion attenuates so as to become a highlight potential while the portion which is not exposed to the laser beam retains the positive charge, thereby having a shadow potential and forming an electrical image, namely, an electrostatic latent image. Thereafter, in the developing portion, positively charged toner is adhered to the highlight potential regions having a low potential. This toner is transferred to paper by applying a negative corona discharge to the back surface of the paper so that the toner thermally and chemically fixed. The toner remaining on the surface of the photosensitive layer without being transferred to the paper is removed with a fur brush and a blade in the cleaning process, and the remaining charges are removed by light or AC static elimination, before proceeding to the next cycle.
At the perforation of a continuous form or between cut sheets, the photoreceptor is directly exposed to the negative corona discharge during transfer, thereby causing negative charge on the photoreceptor. If this negative charge is large, it is difficult to apply a potential for positive charge in the next cycle and the highlight potential and the shadow potential lower, so that a difference in printing density is produced between the photoreceptor between sheets and the photoreceptor covered with a sheet and exerts a deleterious influence on the printing quality.
This negative charge produced by negative corona discharge during transfer is particularly difficult to attenuate on a photosensitive layer of an amorphous selenium material because the mobility of electrons is low therein. From this fact, it is considered that negative charge is attenuated by the mechanism of injecting holes from the substrate, which move to the surface by the action of an electric field, thereby cancelling the negative charges on the surface.
Negative charging can also be a problem in a photoreceptor having a single photosensitive layer or a function separation type photoreceptor consisting of a charge transportation layer (hereinunder referred to as "CTL") on the substrate side and a charge generation layer (hereinunder referred to as "CGL"). The main causes of the negative charging are considered to be:
(1) the degradation of the hole injection by the presence of mainly, an insulating oxide film in the interface between the substrate and the photosensitive layer or the CTL, and
(2) the low hole mobility in the CTL.
Accordingly, it is an object of the present invention to enhance the hole mobility in the CTL so as to solve the above-described problem (2) and to provide an electrophotographic photoreceptor having good characteristics with respect to negative charge.